1. Field of the Invention
The invention relates to an improved method for oxidizing ammonia for the production of nitrogen oxides, which is used in particular in the production of nitric acid and of caprolactam. The invention relates also to an improved system for producing oxidation products of ammonia.
2. State of the Art
In the large-scale production of nitrogen-containing base materials for the chemical industry, the catalytic oxidation of ammonia (NH3) to NOx-nitrogen oxides often forms a base reaction. Mention may be made here of the production of nitric acid (HNO3) as a starting material, for example, for nitrate-containing fertilizers, or of the production of hydroxylamine or hydroxylammonium salts for the production of caprolactam and thus of polyamides.
The following statements concerning the prior art relate by way of example to the production of HNO3 by catalytic oxidation of NH3.
The production of nitric acid is one of the most well-established processes of chemical technology, which was developed to industrial maturity following the introduction of the Haber-Bosch process for NH3 synthesis by W. Ostwald on the basis of platinum catalysts and the design of which still forms the basis of modern HNO3 production even today.
The first commercial system with a platinum catalyst (grooved strips of Pt film) for the production of 1500 tonnes of ammonium nitrate per year was thus constructed in 1906 in Gerthe bei Bochum. A short time later (1909), the first patents relating to the use of woven platinum screens as catalysts appeared. Slightly later again, these were then alloyed with rhodium. Although the catalyst involves high investment costs and is consumed during the NH3 oxidation (platinum burns off), these catalyst systems are still in use today and, in modified form (customized knitted fabrics), still represent the prior art (see Winnacker Michler, Chemische Technik—Prozesse and Produkte, 5th Edition, Volume 3, Chapter 3, p. 248-275, Wiley-VCH Verlag GmbH & Co. KGaA).
Recently, increased use has been made of platinum metal screens with high Pd contents, because they permit not only a certain reduction of costs but also a reduction of the laughing gas (N2O) which is undesirably formed in the NH3 oxidation and which is a greenhouse gas.
Conventional dimensions for platinum metal screens, which are stretched over a wide area in an ammonia oxidation reactor frequently referred to as a “burner”, are in the range of from 0.5 to 5 m diameter. The thickness of the screen packing is conventionally from a few millimeters to approximately two centimeters, depending on the number of screens used.
A gas mixture typically comprising approximately from 9 to 12% by volume NH3 and air flows through the screens, a temperature of approximately from 800 to 950° C. being established at the screens as a result of the exothermic nature of the oxidation reaction. NH3 is thereby oxidized very selectively to nitrogen monoxide (NO) (see reaction scheme 1 below), which is then oxidized in the course of the further process to nitrogen dioxide (NO2) (reaction scheme 2) and finally is converted to HNO3 with water in an absorption tower (reaction scheme 3).
Primary NH3 oxidation—target reaction:4NH3+5O2→4NO+6H2O  (1)NO oxidation:2NO+O2→2NO2  (2)HNO3 formation:3NO2+H2O→2HNO3+NO  (3)The brutto reaction resulting therefrom is:NH3+2O2→HNO3+H2O  (4)
Even though the O2 content of 21% by volume in the combustion air is accordingly just sufficient formally to ensure complete conversion of 10% by volume NH3 to HNO3, in the commercial production of HNO3 further atmospheric oxygen (secondary air) is supplied to the process gas after the catalytic NH3 oxidation and before entry into the absorption tower, in order to accelerate the NO oxidation and thus the formation of HNO3 in the absorption tower. The residual content of oxygen in the waste gas leaving the absorption tower is typically approximately from 1 to 5% by volume.
According to current understanding of the primary oxidation reaction (see Handbook of Heterogeneous Catalysis, 2nd Edition, Volume 5, 2008, Chapter 12.2.7.1, p. 2582, WILEY-VCH Verlag GmbH & Co. KGaA, 2008), a high partial oxygen pressure is required in the combustion of the NH3 in order to suppress the formation of nitrogen and laughing gas, as valueless secondary products, on the surface of the catalyst. This observation is in agreement with the stoichiometries of the formation of N2 and N2O (see reaction schemes 5 and 6 below), which require less oxygen compared with NO formation (reaction scheme 1).
Primary NH3 oxidation—secondary reactions:4NH3+3O2→2N2+6H2O  (5)4NH3+4O2→2N2O+6H2O  (6)
The formation of NO2, which according to reaction scheme (7) would require an increased amount of oxygen, does not take place on platinum metal catalysts.4NH3+7O2→4NO2+6H2O  (7)
The formation of the secondary products, or NOx selectivity, is also dependent on the general operating pressure of the NH3 oxidation. The higher the pressure, the lower the NOx yield. The NOx yields which can be achieved according to the current prior art with different process variants (combustion pressures) are shown in the following table (taken from (Winnacker-Küchler, Chemische Technik—Prozesse and Produkte, 5th Edition, Volume 3, Chapter 3, p. 248-275, Wiley-VCH Verlag GmbH & Co. KGaA).
NH3 content before theCombustion pressurescreenNOx yieldatmospheric12.0-12.5% by volume95-98%Medium pressure (3-6 bar) 9.5-11.0% by volume93-97%High pressure (7-14 bar)10.0-11.0% by volume90-95%
A shortcoming of the platinum metal screen catalysts is, however, the only low stability of the catalyst at the high operating temperature of approximately 900° C. The burning off of the noble metal causes the catalyst to be consumed, at approximately from 0.04 to 0.4 g Pt/t HNO3, according to the prevailing combustion pressure, so that the catalyst must be renewed at regular intervals, approximately every 3 months to 15 months according to the combustion pressure. This leads to not inconsiderable costs, even if a portion of the platinum which has been burnt off is recovered by various catcher systems (e.g. Pd screens).
On account of these disadvantages, attempts have repeatedly been made to develop alternative metal-oxide-based catalyst materials in order in particular to save platinum. An overview of the many different efforts to use oxidic catalysts is given in Sadykov et al., Appl. Catal. General A: 204 (2000), p. 59-87. Thus, especially in Eastern Europe, catalyst systems based on doped iron oxides have been used, often also in combination with platinum metal screens, while in the western hemisphere, cobalt-oxide-based systems have predominantly been employed.
However, none of these attempts at establishing platinum-metal-free NH3 oxidation catalysts has hitherto been able to gain acceptance in industry because such catalysts exhibit lower selectivities of the NO formation as compared with highly selective platinum metal catalysts, and in modern systems for HNO3 production, the product price is determined to the extent of more than 70% by the NH3 price.
In many cases, the potentially active, noble-metal-free transition metal oxide catalysts also experience a considerable deactivation over time under conditions of practice, which is caused not only by sintering effects due to the high thermal stress but often also by a (partial) reduction of the oxides with NH3 to correspondingly lower-valent oxides, which generally exhibit a lower activity and selectivity for NO formation. Mention may be made, for example, of the reduction of MnO2 and Mn2O3 to Mn3O4, the reduction of CuO2 to CuO, the reduction of α-Fe2O3 to Fe3O4 and FeO or, particularly prominently, the reduction of highly active Co3O4 to less active CoO.
In order to counteract such a deactivation, in the case of a commercial use of Co3O4 catalysts for NH3 oxidation in a fixed bed reactor of Incitec Ltd. in Australia, the catalyst bed was periodically rearranged in order to reoxidize with residual oxygen in the rear portion of the catalyst bed the catalyst reduced at a high NH3 concentration in the front portion of the catalyst bed. The same idea is also pursued by corresponding works of Schmidt-Szalowski et al. (see Appl. Catal. A: General 177 (1998), p. 147-157), which publicize the oxidation of NH3 via Co3O4 catalysts in a fluidized bed. The swirling of the catalyst particles is here said to effect in the lower portion of the fluidized bed a continuous reoxidation with oxygen of the CoO that is formed.
A further possibility, which has been investigated many times, for suppressing the deactivating reduction of the oxides is the doping, that is to say stabilization, of the above-mentioned binary oxides with other metal oxides which are difficult to reduce, which is, however, often accompanied by a reduction in the specific activity, as described by Sadykov et al. in Appl. Catal. General A: 204 (2000) p. 59-87. Mention may be made by way of example of the doping of α-Fe2O3 with Al2O3, which formed the basis for the two-stage catalyst systems developed in the 1970s in the USSR for NH3 oxidation in combination with a reduced amount of conventional Pt/Rh screen catalysts. The transition metal oxides can also be converted by doping with other metal oxides into ternary mixed oxides having different crystal structures, in which the higher oxidation states of the transition metals have a reducibility which is in principle low. Mention may be made especially of perovskitic structures, which are distinguished by a high activity for the formation of NO and a high chemical stability.
For example, U.S. Pat. No. 4,812,300 A claims mixed oxide catalysts of the perovskite type having the general formula ABO3±δ for ammonia oxidation, wherein A represents alkali metals, alkaline earth metals, lanthanides or actinides, and B represents one or more elements of groups IB, IVB to VIIB and VIII. The catalysts are said to exhibit an equilibrium oxygen partial pressure of greater than 10−15 bar at 1000° C., so that a good transfer of the lattice oxygen to the NH3 molecule is possible without the structural integrity of the perovskite being impaired. Testing of the catalysts was here carried out in an apparatus or under conditions of temperature-programmed reduction (TPR) at ambient pressure and an NH3 concentration of 3.3% by volume and an oxygen content of 6.7% by volume in helium. Particularly preferred perovskite catalysts comprise lanthanum and/or strontium as the A-position element and cobalt, nickel and/or manganese as the B-position element.
WO-99/25650 A1 describes a device for NH3 oxidation in which there are preferably used mixed oxide catalysts formed of rare earth metals and cobalt. The oxidation of 10% by volume NH3 in air at atmospheric pressure with a lanthanum/cerium/cobalt mixed oxide (atomic ratio La:Ce:Co=8:2:10) is described by way of example.
U.S. Pat. No. 3,888,792 A describes the use of Co3O4 doped with rare earth metals for NH3 oxidation, which is said to have increased selectivity and long-term stability as compared with pure Co3O4. The testing of chosen samples was carried out at an NH3/air volume ratio of 1/10 under atmospheric pressure. In a long-term test over 900 hours with Ce-doped Co3O4, in which an intermediate pressure increase to 7 bar also took place, the yield of NOx was always more than 90%.
WO 2009/028949 A1 claims mixed oxide catalysts for the production of NO by reaction of a gas mixture consisting of NH3 and O2, which catalysts satisfy the general formula A3-xBxO9-y. A and B are selected from metals of the group Mn, Cr, Co, Fe and Al. The catalysts were tested at atmospheric pressure with a gas mixture having a composition of 10% by volume NH3 in air or 10% by volume NH3, 18% by volume O2 and 72% by volume argon. The maximum NOx selectivity achieved of 96% was attained with a mixed oxide having the composition Mn1.5Co1.5O4.
As a further example, mention may be made of U.S. Pat. No. 3,962,138 A. Catalysts for NH3 oxidation which consist of 60-95% Co3O4, 5-15% Al2O3 and 0-25% of an oxide of thorium, cerium, zinc or cadmium are claimed therein. The shaped catalysts were tested in a reactor having a diameter of 10 cm at a pressure of 4-5 bar with a gas mixture comprising 10% by volume NH3 in air. With the best catalysts, each of which contained approximately 10% ThO2, an NOx yield of approximately 93-95% was achieved after an operating time of 400 hours. The addition of Al2O3 and ThO2 brought about a significant improvement in the NOx yield and the lifetime of the catalysts.
DE 10 2012 000 419 A1 discloses a low-temperature oxidation of ammonia in the production of nitric acid by passing an ammonia- and oxygen-containing gas stream over a support layer, heated to less than 500° C., of particles of an LaSrCo oxide catalyst and then cooling the nitrogen-oxide-containing gas stream. This reaction is described by way of example by the reaction of a gas stream which contained 5% by volume carbon dioxide, 5% by volume water, 10% by volume oxygen, 200 ppm ammonia and nitrogen as the remainder.
In WO 2006/010904 A1 there are described oxidation methods which are carried out on selected perovskite catalysts. The catalysts comprise bismuth and/or lanthanides with the exception of lanthanum. The oxidation of ammonia in air is described as a model reaction. DE 199 03 616 A1 describes a method for producing nitrogen oxides having a low degree of oxidation by catalytic oxidation of ammonia in a mixture with air and steam on an oxidation catalyst. Catalysts comprising noble metals or catalysts comprising metal oxides are mentioned.
WO 01/49603 A1 discloses a catalyst comprising cerium oxide and manganese oxide as well as magnesium, aluminum, zinc or calcium oxide, and an activator for the selective oxidation of ammonia with oxygen to dinitrogen oxide N2O. The reaction takes place at relatively low temperatures of 250° C. or below.
In DE 2 148 707 A there is described a catalyst for the oxidation of ammonia to nitrogen oxides. This catalyst consists mainly of cobalt oxide and is characterized by a specific surface area of from 0.1 to 7 m2/g and a volume/weight porosity of from 1 to 15%.
U.S. Pat. No. 5,849,257 describes a method for producing nitrogen oxides in which ammonia is reacted with oxygen in the presence of steam on a copper/manganese oxide catalyst. The catalyst is characterized by a specific X-ray spectrum.
EP 0 384 563 B1 describes a method for oxidizing ammonia in the presence of a cobalt oxide catalyst which has been doped with lithium.
US 2013/0039828 A1 discloses a catalyst structure which is suitable for an ammonia oxidation method and is distinguished by a flexible arrangement of catalyst units. Catalysts can comprise platinum metals or also other metals.
In a scientific publication [J. Catal. 276 (2010) 306-313], Biausque and Schuurmann describe the mechanism of the high-temperature oxidation of NH3 to NO over an LaCoO3 catalyst. To that end, various tests are carried out inter alia with variation of the O2 and NH3 content, wherein in one test series—starting from an NH3 concentration of 3% by volume—the oxygen content was varied between 10% by volume and 40% by volume and in a further test series—starting from an oxygen content of 20% by volume—the NH3 content was varied between 1% by volume and 5% by volume. With regard to the NOx yield achieved, a negative dependence on the O2 partial pressure and a positive dependence on the NH3 partial pressure was found. That is to say, as the O2 partial pressure increases and the NH3 partial pressure falls, an increased formation of N2 and N2O was observed, which is contrary to the known behavior of platinum catalysts for NH3 oxidation.
In Catal. Lett. (2011) 141: 1215-8, Tianfeng Hou et al. describe the catalytic oxidation of ammonia to nitrogen monoxide in the presence of perovskite catalysts of the LaMnO3 and LaVO4 type.
In many of the cases from the prior art cited above, the oxidation of NH3 in air, as is usual in the conventional Ostwald process, is studied or, in the practical examples, a corresponding O2/NH3 volume ratio of at least 1.9 is established. In almost all cases, the studies or published data are additionally limited to atmospheric conditions, which yield significantly higher selectivities of the NO formation than are to be expected for elevated pressures.
Nevertheless, the high benchmark of NOx yields set with Pt/Rh screen catalysts is not achieved. This is the case in particular also with high throughputs of NH3, that is to say at a high starting concentration of 10% by volume and elevated operating pressure, which are advantageous and conventional for commercial operation owing to the resulting smaller apparatus sizes and optimal adaptation to the subsequent NO/NO2 absorption. The yield of NOx is thus usually reduced at a high concentration or high (partial) pressure of ammonia. This is the case in particular for known oxide-based catalysts such as, for example, Co3O4 (see, for example, Andrew, S. P. S.; Chinchen, G. C., “The loss in selectivity of a cobalt oxide ammonia oxidation catalyst” in “Studies in surface science and catalysis”; 6 (1980), p. 141-148, (Catalyst deactivation: proceedings of an international symposium, Antwerp, Oct. 13-15, 1980)), which, compared with metal platinum-based catalysts, exhibit a significantly lower activity. A high partial pressure of ammonia promotes to an enhanced degree undesirable secondary and subsequent reactions, which lead to the formation of N2 or also N2O.
Despite various efforts, transition metal oxide catalysts for NH3 oxidation therefore play no role in large-scale applications, apart from the mentioned occasional combination of iron-oxide-based catalysts with noble metal screens.
Pt/Rh screen catalysts are still used almost without exception. As mentioned above, according to the operating pressure of the NH3 combustion (atmospheric/medium pressure/high pressure) and the prevailing pressure level of the NOx absorption in the absorption tower, it is possible to distinguish between different method or system variants (see also Winnacker-Küchler, Chemische Technik—Prozesse and Produkte, 5th Edition, Volume 3, Chapter 3, p. 248-275, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2005; Thiemann, M., Scheibler, E., Wiegand, K. W. Nitric Acid, Nitrous Acid, and Nitrogen Oxides, Wiley-VCH Verlag GmbH & Co. KGaA, 2000).
Of importance today are especially the so-called single or mono-pressure method, in which medium pressure or high pressure are used both for the NH3 combustion and for the NOx absorption, and the so-called dual pressure method with NH3 combustion under medium pressure and NOx absorption under high pressure. The previously conventional systems with combustion at atmospheric pressure and medium-pressure absorption have today largely been superseded by the single pressure and dual pressure methods, which are more economical in the case of larger capacities.
FIG. 1 shows a simplified flow diagram of a typical mono-medium pressure system.
Systems for producing HNO3 thus typically comprise an NH3 evaporator for providing gaseous NH3, an air compressor for the combustion air, an NH3 oxidation reactor for receiving the Pt screen catalysts with an integrated process gas cooler, various heat exchangers or coolers and condensers for further cooling the process gas or for heating the residual gas leaving the absorption tower, an absorption tower for absorbing NOx and forming HNO3, a reactor for the (catalytic) removal of residual NOx and optionally N2O contained in the residual gas, and a residual gas turbine for energy recovery upon expansion of the residual gas into the atmosphere. In dual pressure systems, an additional compression stage for compressing the process gas to the desired absorption pressure is arranged between the NH3 oxidation reactor and the absorption tower.